HISTORIC VARIATION IN SURFACE WATER SLOPE OF THE MIDDLE MISSISSIPPI RIVER

Records of historical changes (late-1800’s to 2001) in surface water slope were developed using available stage data from six United States Army Corps of Engineers gauging stations on the middle Mississippi River between Saint Louis and Thompson Landing, Missouri. Surface water slope was calculated for five reaches (Saint Louis to Chester, Chester to Grand Tower, Grand Tower to Cape Girardeau, Cape Girardeau to Commerce, and Commerce to Thompson Landing). Surface water slope was derived by dividing the difference in stage readings by reach length. The maximum stage was selected at each gauge for each year of observation. The reach length was interpolated from available surveys (1884, 1908, and 1939) or measured from navigation maps.

Analysis of surface water slope for the Saint Louis to Chester and Chester to Grand Tower reaches reveal increasing trend in slope from approximately 1890 to 1910 and then a decrease until present (2001). The Grand Tower to Cape Girardeau and the Commerce to Thompson Landing reaches show an increasing trend in slope through the period of record (1897 - 2001). The Cape Girardeau to Commerce reach shows no trend through the period of record (1896 - 2001).

The investigation into the causal mechanisms of historic trends in surface water slope is currently on going. However, the reaches with decreasing trends in slope occur within the greatest concentration of wing dams on the middle Mississippi River. In addition the reach with no trend in slope occurs in a bed rock-floored section of the river. The variability of surface water slope between reaches coupled with these preliminary observations suggests a complex relationship between slope, natural processes, and river engineering.